1. Field of the Invention
This invention relates to an automatic focusing device of a camera, and more particularly to an automatic focusing device best suited for an automatic focusing camera in which light passed through a picture-taking lens is metered and the picture-taking lens is driven to the in-focus position on the basis of the metered value.
2. Description of the Prior Art
A variety of so-called TTL type automatic focusing devices in which the focusing operation is effected on the basis of such metered value of the light passed through a picture-taking lens have heretofore been proposed. For example, there is known a system in which light-receiving devices are disposed in front of and behind the predetermined focal plane of the picture-taking lens and a front focus signal (a signal representing that an object image is formed forwardly of the predetermined focal plane), a rear focus signal (a signal representing that an object image is formed rearwardly of the predetermined focal plane) and an in-focus signal (a signal representing that an object image is formed substantially on the predetermined focal plane) are obtained from the difference between the outputs of these light-receiving devices and the picture-taking lens is driven to the in-focus position by these focus detection signals; a system is also known in which a pair of object images formed by light beams passed through two different areas of the picture-taking lens are detected by light-receiving devices, the relative position of the two object images is photoelectrically detected and then front focus, rear focus and in-focus signals are obtained to thereby effect focusing operation. None of these systems can obtain a proper focus detection signal when the object image is of low contrast. There is also a system in which, aside from the contrast of the object image, a proper focus detection signal cannot be obtained if the picture-taking lens is greatly spaced apart from the in-focus position. Low contrast of the object image occurs when the object to be photographed itself is of low contrast like a black board or a white board, for example or when the object itself is of high contrast but the object image is extremely blurred because the picture-taking lens is greatly spaced apart from the in-focus position. In the former case, a focus detection signal cannot originally be obtained and automatic focusing operation is impossible, but in the latter case, the picture-taking lens can be driven between the close distance position (the position of the picture-taking lens when its focus is adjusted to an object at the closest distance) and the infinity position (the position of the picture-taking lens when its focus is adjusted to an object at infinity), namely, over the entire movement range of the picture-taking lens, whereby the picture-taking lens can be brought close to the in-focus position and therefore, a focus detection signal can be obtained and automatic focusing becomes possible. It is disclosed in U.S. Pat. No. 4,303,321 that in case a focus detection signal cannot be obtained because of the low contrast of the object image, the picture-taking lens is forcibly driven over said entire movement range to thereby obtain a focus detection signal.
In case said low contrast is attributable to the low contrast of the object itself, however much the forcible driving of the picture-taking lens may be effected, no focus detection signal can be obtained after all and this means wasteful consumption of the power source. To avoid this, it is desirable to stop the driving of the picture-taking lens when a focus detection signal is not obtained in spite of the picture-taking lens having been driven over at least said entire movement range. However, in case the driving of the picture-taking lens has been so stopped, there arises a problem that even if the object later changes to a high contrast object, focus detecting operation cannot take place immediately thereafter.
Although reference has been made to the TTL type automatic focusing device in the foregoing, the above-noted inconveniences are not limited thereto but generally apply to an automatic focusing device using a focus detecting device which cannot obtain a focus detection signal depending on the object to be photographed.